Integrated circuits do fail...ever had a memory card suddenly stop working? Other components soldered onto the boards fail, like resistors and capacitors. So even though a very small number of components on these circuits fail in infancy, more do fail at later times, too. The life statistics on anything electronic show this to be true, although we are much more accustomed to something mechanical or electro-mechanical to fail. electronics do die suddenly. In my case, a 5Dc suddenly stopped working in the middle of a shoot, after two shots had been taken moments earlier, so it was not even associated with a power-up per se (as BigAl007 had mentioned)...it went to sleep and would not wake up again. Simply would not turn on at all, the LCD on top showed nothing, in the viewfinder showed nothing, no little winking LED near the power switch when it was turned on.
I found an independent repair shop still working on the 5D (although Canon had discontinued servicing them) and the main circuit had to be replaced. It happens...in your case it was coincident with the Canon sensor cleaning visit. Canon could have run a diagnostic test as a routine check on the camera, part of an overall condition check which included a test of shutter speed timing circuit and the mechanical actions of the shutter, and the diagnostic found an issue when it cycle one of the circuits thru multiple cycles of a test. My computer runs a self test on itself on a regularly timed interval just sitting in my home, and I am glad to see a healthy computer report each time; it happened that your camera failed a test suite while it was in for shutter cleaning.
Our Canon electronic flashes fail, the trigger circuits fail, the charging circuits fail, the main circuit fails...things electronic fail suddenly.